A conventional perfect binding machine is provided with a conveying path, a series of binding units arranged along the conveying path for performing perfect binding, and at least one clamp unit movable along the conveying path. In this perfect binding machine, a book block, which consists of a plurality of sheets or signatures, is held between a pair of clamp plates of the clamp unit and then, while the book block is conveyed by the clamp unit along the series of binding units, the perfect binding is performed. Furthermore, in this perfect binding machine, the respective two or more binding units have a pair of members with which the book block is engaged at its both sides protruding from between the pair of clamp plates, a thickness measurement unit for measuring a thickness of the book block. Thus, prior to start of the perfect binding, the thickness of the book block is measured by the thickness measurement unit so that a gap between the pair of clamp plates and a gap between the pair of members of the respective binding units are adjusted based on the measured value of the thickness of the book block (See, for example, Patent Document 1).
FIG. 7 are perspective views schematically showing such perfect binding machine. Referring to FIG. 7, the perfect binding machine includes a conveying path F, a series of binding units (a milling unit B, an adhesive application unit C and a cover attachment unit D) which are arranged along the conveying path F for performing the perfect binding, a single clamp unit (in FIG. 7, only clamp plates 1a, 1b of the clamp unit are shown) arranged for reciprocal movement along the conveying path F, and a first drive mechanism (not shown) moving the clamp unit along the conveying path F.
When the perfect binding is started, at a book block insertion position A, a book block P is inserted between the pair of clamp plates 1a, 1b of the clamp unit and placed on a alignment plate 1c in such a manner that a back of the book block P faces downwardly. Then the book block P is held between the clamp plates and conveyed by the clamp unit toward the milling unit B along the conveying path F.
The milling unit B has a milling cutter 2a and a pair of guide plates 2b, 2c. While the book block P passes the milling cutter 2a, both sides of the book block P protruding between the pair of clamp plates 1a, 1b passes between the pair of guide plates 2b, 2c. Thus the back of the book block P is cut while the book block P is supported by the pair of guide plates 2b, 2c at the both sides thereof. After that, the book block P is conveyed to the adhesive application unit C by the pair of clamp plates 1a, 1b. 
The adhesive application unit C has an adhesive tank 3a storing an adhesive, an adhesive applying roller 3b, and a roller 3c wiping off an excessive adhesive. The adhesive application unit C applies the adhesive of an appropriate thickness to the back of the book block P. When the application of adhesive is completed, the book block P is conveyed to the cover attachment unit D by the pair of clamp plates 1a, 1b. 
The cover attachment unit D includes a bottom plate 4c and a pair of nip plates 4a, 4b. When the perfect binding is started, a printed cover Q is fed from a cover supply unit (not shown) onto the bottom plate 4c and the pair of nip plates 4a, 4b in a direction of an arrow S by an appropriate feed means such as a conveyor. Thereafter the book block P held between the pair of clamp plates 1a, 1b stops at a position where the back of the book block P is opposed to a corresponding back of the cover Q. Then the bottom plate 4c and the pair of nip plates 4a, 4b rise and the cover Q is pressed against the back of the book block P by the rising bottom plate 4c and at the same time, the movable nip plate 4a moves toward the stationary nip plate 4b so as to press the cover Q against the both sides of the book block P, thereby the cover Q is adhered to the book block P and a bound product is produced. After that, the clamp unit returns to the book block insertion position A, and the pair of clamp plates 1a, 1b open to a maximum extent to discharge the bound product.
The perfect binding machine includes a thickness measurement unit for measuring a thickness of the book block P. FIG. 8A is a plan view of the thickness measurement unit, and FIG. 8B is an elevation view of the thickness measurement unit shown in FIG. 8A.
As shown in FIG. 8A and FIG. 8B, the thickness measurement unit includes a base 40a, a pair of support members 40b fixed to the base 40a and spaced from each other, and a horizontal linear guide rail 40c extending between the support members 40b and supported by the support members 40b. 
The thickness measurement unit further includes a slidable body 40d slidably mounted on the guide rail 40c. The slidable body 40d has a flat measuring surface 40e extending perpendicularly to the guide rail 40c. 
The thickness measurement unit further includes a stationary body 40f fixed to the base 40a. The slidable body 40d slides in directions toward and away from the stationary body 40f, and the stationary body 40f has a flat reference surface 40g which is opposed to and able to make contact with the measuring surface 40e of the slidable body 40d. 
The thickness measurement unit further has travel distance measurement unit for measuring a travel distance of the measuring surface 40e from the reference surface 40g. A zero point of the travel distance is established as a point at which the measuring surface 40e of the slidable body 40d contacts the reference surface 40g of the stationary body 40f. The travel distance measurement unit has a pair of pulleys 40h, 40i which are arranged at both ends of the guide rail 40c and attached to the base 40a, and an endless belt 40j extending between the pulleys 40h, 40i. The slidable body 40d is fixed to the endless belt 40j. The travel distance measurement unit further has a rotary encoder 40k coupled to a rotational shaft of the pulley 40h. 
The thickness measurement unit is arranged adjacent to a table for jogging the book block P. The table is arranged adjacent to the book block insertion position A of the binding machine. In this case, the thickness measurement unit has an auxiliary table element 40m coupled to the table in such a manner that the auxiliary table element 40m is flush with the table. The auxiliary table element 40m is provided with a slot (not shown) extending along the guide rail 40c, the base 40a is fixed to a lower surface of the auxiliary table element 40m, at least the reference surface 40g of the stationary body 40f and the measuring surface 40e of the slidable body 40d protrude from an upper surface of the auxiliary table element 40m through the slot, and the measuring surface 40e is arranged for slide movement.
Prior to start of the binding operation of the binding machine, the book block P is jogged on the table by the operator and then, inserted between the reference surface 40g and the measuring surface 40e of the thickness measurement unit.
Next, the operator supports the book block P in a standing position with his (or her) one hand and slides the slidable body 40d toward the stationary body 40f with his (or her) other hand, so that the book block P is pressed by the measuring surface 40e of the slidable body 40d against the reference surface 40g of the stationary body and a thickness of the book block P is measured.
On the basis of the obtained measured value, a gap between the pair of clamp plates 1a, 1b before holding a book block, a gap between the pair of guide plates 2b, 2c of the milling unit B, a gap between a pair of laterally adhesive applying rollers 3c, 3d of the adhesive application unit C, a gap between the pair of nip plates 4a, 4b of the cover attachment unit D, and a gap between a pair of crease forming rollers 5a, 5b of a cover feeding unit E are adjusted by a control unit so that these gaps are adapted to the thickness of the book block P.
However, in such perfect binding machine, at measurement of the thickness of the book block P, the operator has to keep the book block P in the standing state with his (or her) one hand, while moving the slidable body 40d with his (or her) other hand. Consequently, in manufacturing various kinds of bound products in small quantities, the measurement of the thickness consumes a lot of time, which leads to low productivity. Further, for a thick book block P, it is difficult for the operator to support the book block P in a standing position only with his (or her) one hand and therefore, he (or she) often loosed the book block P while moving the slidable body 40d with his (or her) other hand, so that he (or she) had to restart the measurement from the beginning. Thus, the measurement of the thickness was burdensome for the operator.
In addition, it is necessary to press the book block P against the stationary body 40f with the slidable body 40d on measuring the thickness. However, the pressing force is not uniform because the book block P is manually pressed. Consequently, for a thick book block P, it is not possible to sufficiently remove air from the book block P, so that a measurement error increases, which leads to reduction in quality of the bound product.